1. FIELD OF THE INVENTION
This invention relates to an active type electro-optical device, particularly an active type liquid crystal device, and more particularly to a display device in which two thin-film type insulated gate field effect transistors (hereinafter referred to as "TFT") of P-channel and N-channel types are complementarily provided to each picture element to thereby form pixels on a screen of the device.
2. DESCRIPTION OF THE PRIOR ART
An active type of liquid crystal device using TFTs has been conventionally known as an effective display device. In this type of the display device, the TFT is formed of semiconductor of amorphous or polycrystal structure, and each picture element of the device is equipped with only one of two conducting types (P-channel and N-channel types) of TFTs. In general, an N-channel type of TFT (hereinafter referred to as "NTFT") is connected to each picture element in series, as representatively shown in FIG. 6.
The active matrix type of liquid crystal display device as described above generally has many picture elements, for example, 480.times.640 or 1260.times.960 picture elements. FIG. 6 shows an example of the active matrix type of liquid crystal display device as described above, where a 2.times.2 matrix arrangement is illustrated in order to simplify the description thereof. In this matrix arrangement, plural gate lines G.sub.1 and G.sub.2 and plural signal lines D.sub.1 and D.sub.2 are arranged in Y-axial and X-axial directions respectively so as to be intersected to each other, and a displaying element for a picture element is provided at each intersecting point. Each displaying element for the picture element comprises a liquid crystal portion 102 and TFT portion 101. Each picture element is supplied with signals from peripheral circuits 106 and 107 to selectively switch desired picture elements between on and off states for a displaying operation.
In a displaying operation of the liquid crystal display device which has been actually manufactured, there frequently occurs a case where an output voltage of the TFT, that is, an input voltage 100 (V.sub.LC) to the liquid crystal portion 102 (hereinafter referred to as "liquid crystal potential") is not set to "1" (high state) when it must be set to "1", and also is not set to "0" (low state) when it must be set to "0". This unfavorable phenomenon is caused by an asymmetric electric characteristic of a switching element (that is, TFT) for supplying a signal to the picture element. That is, the TFT has an asymmetric electrical characteristic between charging and discharging actions thereof to and from a picture element electrode which corresponds to one electrode of the liquid crystal portion 102. The liquid crystal has originally an insulating property in its action, and thus the liquid crystal potential V.sub.LC is left floated (undischarged) during the Off-state of the TFT. The liquid crystal portion 102 equivalently corresponds to a capacitor, and thus the liquid crystal voltage V.sub.LC is determined by charges accumulated in the liquid crystal portion 102. The charges accumulated in the liquid crystal portion 102 is liable to leak therefrom in a case where the resistance of the liquid crystal portion 102 is reduced to a relatively small value due to R.sub.LC or where dust or ionic impurities exist in the liquid crystal portion 102, or liable to leak from pinholes which have been formed in a gate insulating layer of the TFT, so that the liquid crystal potential V.sub.LC is unstably varied and thus is set to an incomplete value. Therefore, for the liquid crystal display device having a large number of picture elements of 200,000 to 5,000,000, high yield can not be obtained.
In addition, the leaking of charges accumulated in the liquid crystal portion 102 is also caused between neighboring picture elements due to the following rubbing process. That is, a TN (twisted nematic) liquid crystal is generally used as the liquid crystal portion 102. In order to orient liquid crystal molecules to a predetermined direction, both electrodes of the liquid crystal portion 102 for driving the liquid crystal are provided with orienting films which have been subjected to a rubbing process. The liquid crystal portion 102 is weakly broken down by static electricity which has generated in the rubbing process, so that the charges accumulated in the liquid crystal portion 102 are liable to leak between neighboring picture elements or between neighboring conducting leads through the broken-down portion, or leak through a damaged gate insulating layer.
In the active type liquid crystal device as described above, it is very important to keep the liquid crystal potential V.sub.LC to a predetermined level (the same as an initial level) at all times during one-frame displaying operation. However, an actually-manufactured display device has some malfunctioning active elements, so that the liquid crystal level can not be actually kept to the initial level during one-frame displaying operation.
In order to overcome the above disadvantage, there has been proposed a display device as shown in FIG. 7 in which each picture element is provided with a matrix wiring comprising a pair of first and second signal lines in the X-axial direction and a third signal line in the Y-direction, and a complementary thin film transistor and a picture element electrode are provided at an intersecting portion of these signal lines. In the display device thus constructed, as shown in FIG. 7, the source portions of a P-channel thin film transistor 2 and an N-channel thin film transistor 1 are commonly connected to a picture element electrode, and the drain of the N-channel thin film transistor 1 is connected to the first signal line 35 of the paired signal lines in the X-direction while the drain portion of the P-channel thin film transistor 2 is connected to the second line 11. On the other hand, gate portions of the respective P-channel and N-channel thin film transistors are commonly connected to the third signal line 33.
According to the display device as shown in FIG. 7, the foregoing disadvantage can be overcome, and a current margin is more increased, that is, a response speed is more increased. In addition, the potential of picture element at each pixel, that is, the liquid crystal potential V.sub.LC is sufficiently stably set to "1" or "0", and thus the level of the voltage V.sub.LC is prevented from drifting during one-frame displaying operation.
However, the display device as described above has a circuit construction that three signal lines are required for each picture element, and such a large number of signal lines causes reduction of an aperture ratio for determining display quality of the display device. This is a critical problem to the display device.